Textile substrates need various chemical treatments depending on the desired properties of the end-uses. Chemical treatment for textile substrates can be done either by batch or continuous process. For a batch process, a specific amount of textile substrate is treated with chemical formulations for a specific period of time. The amount of chemicals used is normally based on the amount of the substrate being treated or on the amount of formulation being used. In general, in batch processes the exact amount of chemical being used is calculated based on either total amount of the textile substrate or formulation, which is expressed as “% owg (on weight of goods)” or “% owb (on weight of bath)”, respectively.
For continuous textile wet processes, textile substrates are treated continuously by being passed through one or more process steps arranged in tandem. Textile substrates pass through a chemical formulation in a treatment bath and the completely soaked substrates pass through a pair of squeeze rolls to remove excess amounts of the formulation in order to control the amount of chemical formulation on the substrates. Then, the substrates continue to pass through a drying (e.g, heating) stage, such as an oven, to remove residual water and to fix the chemicals on the substrates. The amount of chemicals applied on textile substrates depends on the concentration of chemicals in the formulation and the “wet pickup”. Wet pickup is the amount of the chemical formulation picked up by the substrate and is expressed as a percentage on weight of the dry substrate. The wet pickup on the substrate is controlled by the nip pressure of the squeeze rolls. To give uniform chemical distribution throughout or over the substrate, the wet pickup must be controlled evenly across the width and along the length of the substrate.
Most chemical treatments for textile substrates are performed at the “fabric stage” (e.g., a stage where the fabric has already been produced from fibers). However, chemical treatments are also performed at the “fiber stage” (e.g., when chemical-treated fibers are required for yarn spinning or nonwoven production). For a batch process chemical treatment of fibers, a specified amount of loose fibers is loaded in a perforated basket, and the basket is loaded into a chemical treatment device such as a stock dyeing machine. After loading the basket, a specific amount of chemicals is applied on the fibers using the dyeing machine or other chemical treatment device for a specified period of time. In contrast, in a continuous process, the fibers in a web or batt form are continuously passed through one or more process steps arranged in tandem. The wet pickup control for the fibers is difficult in a continuous process when compared to woven fabrics because the thickness of the fiber web (or batt) is generally uneven across the width and along the length.
If, in a continuous process, the fibers were subjected to scouring, bleaching, and rinsing, the fibers will contain only water after final squeezing. In this case, even though there will be a variation of wet pickup on the fibers, this will generally not pose a problem since there will be no remaining chemical on the fibers after drying. In sharp contrast, when the fibers are subjected to chemical formulation treatment, the wet pickup variation will cause uneven chemical distribution throughout the final dried fibers. This will cause an uneven quality (property) on the final products (yarn or nonwoven) made with these fibers.
U.S. Pat. Nos. 4,213,218, 4,425,842, and 4,944,070, each of which are herein incorporated by reference, describe methods of continuous wet finishing for fibers. These applications require the loose fibers to be converted into a web or batt form before the wet treatment. These applications utilize a squeezing system to control final chemical amount on the treated fibers. In operation, the fiber web (or batt) soaked with a chemical formulation is passed through a pair of squeeze rolls. The amount of the chemical formulation picked up by the fibers is controlled by the pressure of the squeeze rolls. However, in practice, the squeezing system does not provide an even chemical distribution on the final treated fiber because the thickness of the fiber web (or batt) squeezed is not even. The thickness of fiber web (or batt) is much less controllable compared to the thickness of woven fabrics.